As methods of digitizing and transmitting (recording and reproducing) an audio signal, there have hitherto been proposed recording and reproducing apparatus, such as a CD (compact disc) player and a DAT (digital audio type recorder) and digital audio broadcasting such as broadcast satellite or the like. The digital audio transmitting apparatus uses a format in which a sampling frequency is 48 kHz or 44.1 kHz and a quantization bit number is 16 bits or the like in order to digitize an audio signal.
In the digital audio transmitting apparatus, the quantization bit number of the digital audio signal generally limits a dynamic range of a demodulated audio signal. Therefore, when data of high audio quality is transmitted, the quantization bit number has to be expanded from the existing bit number, i.e., 16 bits to another bit number, such as 20 bits or 24 bits. Once the format is determined, it is not so easy to expand the quantization bit number. Thus, the digital audio transmitting apparatus cannot generate an audio signal of high audio quality.
As a method of digitizing an audio signal, there is proposed a method which is often referred to as sigma-delta modulation. Such a method is disclosed in U.S. Pat. No. 5,351,048 (issue date: Sep. 27, 1994) to Yoshio Yamasaki. The disclosure of U.S. Pat. No. 5,351,048 is hereby incorporated by reference.
FIG. 1 of the accompanying drawings shows in block form an arrangement of an example of 1-bit sigma-delta modulation. As shown in FIG. 1, an input audio signal applied to an input terminal 91 is supplied through an adder 92 to an integrator 93. An integrated output signal from the integrator 93 is supplied to a comparator 94, in which it is compared with a mid-range potential of the input audio signal and quantized in one bit at every sample period. As a frequency (sampling frequency) of the sample period, there is used a frequency which is 64 times or 128 times as high as 44.1 kHz or 48 kHz (2.816 MHz. & 5.632 MHz or 3.072 MHz & 6.144 MHz).
The quantized signal is supplied to a delay circuit 95 and thereby delayed by one sample period. The delayed signal is processed through a one-bit digital-to-analog (D/A) converter 96 to an adder 92, in which it is added with the input audio signal supplied thereto from the input terminal 91. The quantized signal output from the comparator 94 is developed at an output terminal 97. Therefore, according to the sigma-delta modulation, as described in the above-mentioned literature, it is possible to obtain a digital audio signal of wide dynamic range by sufficiently increasing the frequency (sampling frequency) of the sample period.
In the sigma-delta modulation, when the modulated (quantized) signal is transmitted (recorded and reproduced), "+1" of the quantized signal is converted to "1" and "-1" of the quantized signal is converted to "0". In that case, if an abnormality occurs in the transmission system and the signal is lost, then the signal is fixed to "1" or "0". In the sigma-delta modulation, consecutive "1" and "0" correspond to a positive maximum value and a negative maximum value of a demodulated signal, respectively. Accordingly, if a signal line is broken in the transmission system, then a noise of maximum level occurs at that very moment. As a result, the risk of damage to a monitor amplifier or loudspeaker due to excessive input signal is quite great.
In a CD player or DAT, for example, the signal format is determined so that the consecutive "1" and "0" become intermediate values of the demodulated signal, respectively. Thus, the occurrence of the noise of maximum level can be avoided. It is therefore proposed that data, which results from the sigma-delta modulation, is converted to data conforming to the signal format of the CD player and the DAT by using a decimation filter. In this case, however, it becomes difficult to extract data of high audio quality from the transmitted signal by increasing the quantization bit number.